Plastic films with improved properties are constantly demanded by industry. For example, in order to save costs and decrease the amount of material that must be recycled or put in a landfill, downguaged (that is thinner) films are desired. Successful downguaging requires the use of a resin with a relatively high modulus to accommodate the gauge reduction without a significant loss of productivity in converting operations and customer acceptability. As used herein, “modulus” refers to the stiffness of the film which is indicated by the 2% secant modulus as measured by ASTM D 882. Likewise, film manufacturers desire resins that can run at high production rates. Such resins require relatively high melt strength to provide web or bubble stability prior to quenching. Furthermore, many applications require the film to have good toughness (that is, high Elmendorf tear strength, dart impact and/or puncture values). In addition, for shrink film applications, the film should have a high degree of shrink (40-80%) in the machine direction and positive shrink (10-30%) in the cross direction.
Blown films are typically fabricated from ethylene polymers (also known as polyethylene (PE)). Different classes of ethylene polymers provide different film properties. Generally, selecting optimum performance is a matter of trading off one property against another, for example, increasing modulus decreases toughness. For instance, linear low density polyethylenes (LLDPE) provide good toughness and other desirable properties but these properties decrease as the modulus (modulus is proportional to density for polyethylenes) of the LLDPE increases. As such, the relatively low modulus of LLDPE limits the possibilities of downguaging the film. Moreover, LLDPE can be difficult to process at high rates, especially when run neat, due to insufficient melt strength. Also, LLDPE films generally have little cross directional shrinkage in conventional blown films. While the addition of low density polyethylene (LDPE) to the LLDPE provides improved processability (by increasing melt strength) and cross directional shrinkage, the presence of the LDPE can diminish the physical properties of the LLDPE. LDPE resins provide better processability, but generally compromise other properties, such as toughness, and do not enhance modulus. This reduced toughness limits the possibility of downguaging the film.
High molecular weight (for example, MI<0.1) polyethylene (HMWPE), which is typically a high density (>0.945 g/cc) polymer, exhibits high melt strength and high modulus in blown films. However, such a high melt strength HMWPE typically produces a film with very low tear resistance. While the impact strength of these HMWPE films can be improved via specific processing conditions, such as using a high blow-up ratio, the tear resistance remains poor. Blending LLDPE is often practiced to enhance sealability and tear resistance of HMW HDPE, but practically, such blends are limited to minor amounts of LLDPE. Conversely, HMW HDPE is sometimes blended into LLDPE films to improve creep resistance, but the melt blending quality and resultant melt orientation properties of the blend on LLDPE blown film equipment can be challenging.
In contrast to polyethylene resins, polypropylene (PP) resins have a relatively high modulus. However, PP resins have poor processability due to low melt strength and also have poor film toughness properties. Additionally, polypropylene and polyethylene are immiscible and, as such, are generally considered to be incompatible with each other for film forming. The compatibility of PP resins and PE resins can be improved somewhat by using PP impact copolymers (ICP) as the PP resin. However, even ICPs are not fully miscible with PE resins. This inherent incompatibility would be expected to severely limit the physical properties of a blend of PE and PP. Moreover, an ICP resin generally does not have substantially improved melt strength over other PP resins.
Therefore, a resin is still needed that provides the good processability of LDPE and the high modulus of a PP resin while delivering the desirable physical properties of an LLDPE resin. Surprisingly, the applicants have found that blending a minor proportion of a rheology modified (also known as “coupled”) PP resin into a major proportion of an LLDPE resin yields a film with a high modulus and which also maintains good toughness.